The present invention relates to a method of manufacturing a silicon single crystal, which is suited for use in the manufacture of a semiconductor integrated circuit device, by the Czochralski method and to a method of manufacturing an epitaxial wafer from the silicon crystal produced by that method. More particularly, it relates to a method of manufacturing an epitaxial wafer exhibiting high gettering capability while scarcely giving rise to stacking faults, dislocations and other defects in an epitaxial layer (hereinafter collectively referred to as xe2x80x9cepitaxial layer defectsxe2x80x9d) when it is grown on a wafer obtained from a silicon single crystal produced with or without doping with nitrogen, and to a method of producing such a single crystal to serve as a raw material for the epitaxial wafer.
With the recent increase in the integration density of silicon semiconductor devices, quality requirements imposed on silicon wafers on which devices are formed have become more and more severe. For example, severer limitations are imposed than ever on dislocations and like crystal defects and/or metal impurities in the so-called xe2x80x9cdevice active regionxe2x80x9d where devices are formed, with the increasing fineness of circuits as resulting from the increase in integration density, since such defects and impurities increase the leakage current and shorten the life of a carrier.
In the art, wafers prepared by slicing silicon single crystals produced by the Czochralski method are used for forming semiconductor devices thereon. These wafers generally contain about 1018 atoms/cm3 of supersaturation oxygen. Due to the thermal history in the steps of device formation, such oxygen forms oxide precipitate nuclei and thereby induces crystal defects such as dislocations and stacking faults. In the process of device manufacture, however, the so-called DZ layer (denuded zone) which is free of crystal defects and has a thickness of about tens of micrometers is formed near the wafer surface by diffusion of oxygen to the outside when the wafer is maintained at about 1100xc2x0 C. for several hours in the step of field oxide film formation by LOCOS (local oxidation of silicon) or well diffusion layer formation. The DZ layer serves as a device active region, so that the occurrence of crystal defects is spontaneously prevented.
However, with the increase in the integration density of semiconductor devices, the high-energy ion implantation technique has been introduced for well formation by which the device process is carried out at a low temperature of 1000xc2x0 C. or less. At such a temperature, the above-mentioned outward diffusion of oxygen does not occur to a sufficient extent, hence the DZ layer formation near the surface becomes insufficient. Therefore, attempts have been made to reduce the oxygen content in wafers, but such attempts have been unsuccessful in perfectly suppressing the formation of crystal defects.
Under such circumstances, epitaxial wafers having an epitaxial layer substantially free of crystal defects as formed therein have been developed and are now widely used in the manufacture of highly integrated devices. However, even epitaxial wafers high in crystallinity are used, the device characteristics are degraded due to contamination of the epitaxial layer with metal impurities in the subsequent device process steps.
The opportunity and influences of such contamination with impurity metal elements increase since the process becomes more complicated with the increase in the integration density. The contamination may be eliminated basically by cleaning the process environment and materials used. However, it is difficult to render the device process completely free of contaminants, hence the gettering technology becomes necessary as a measure for solving that problem. This is a means for entrapping contaminant impurity elements in a region (sink) other than the device active region to thereby render the contaminants harmless.
The gettering technology includes intrinsic gettering (hereinafter referred to as xe2x80x9cIGxe2x80x9d for short) which comprises entrapping impurity elements by utilizing oxygen-caused oxide precipitates spontaneously induced during heat treatment in the device process steps. However, when a wafer is heat-treated at a temperature as high as 1050-1200xc2x0 C. in the epitaxial step, oxide precipitate nuclei occurring within the wafer sliced from a silicon single crystal shrink and vanish, whereby it becomes difficult to sufficiently induce oxide precipitates to serve as gettering sources within the wafer in the subsequent device process steps. Thus, even if the gettering technology is applied, a problem arises that any satisfactory IG effect cannot be exerted on metal impurities throughout the whole process.
To overcome such a problem, there is a proposal in the art that the wafer be heat-treated at 600-900xc2x0 C. prior to the epitaxial step in the device manufacture to thereby allow oxide precipitate nuclei to grow to such a size that they hardly vanish even upon high temperature treatment in the epitaxial step (cf. e.g. Japanese Patent Application Laid-Open (Kokai) No. H08-339024).
Specifically, according to the proposal, oxide precipitate nuclei in the crystal are increased in size by heat treatment prior to device treatment to thereby increase the thermal stability thereof to a sufficient level. Even when the wafer is thereafter subjected to a high temperature heat treatment in the epitaxial step, the oxide precipitate nuclei will not shrink or vanish. The oxide precipitate nuclei that have survived the epitaxial step form oxide precipitates from the early stages of the device step and thus effectively serve as sinks for gettering, so that an excellent gettering effect can allegedly be expected. However, the proposed method has a problem in that the above heat treatment is required as a new process step in the silicon wafer manufacturing process and increases the cost of production of epitaxial wafers.
In addition to the above measure, methods of producing silicon single crystals have been proposed in the art which comprise doping the single crystals with nitrogen while they are grown by the Czochralski method, to thereby induce formation, within wafers, of oxide precipitate nuclei hardly vanishing even upon high temperature heat treatment in the epitaxial step (cf. e.g. Japanese Patent Application Laid-Open (Kokai) No. H11-189493 and Japanese Patent Application Laid-Open (Kokai) No. 2000-44389).
According to the methods proposed, a silicon single crystal having oxide precipitate nuclei which hardly shrink or vanish can be obtained even in the epitaxial step by increasing the thermal stability of oxide precipitate nuclei in the crystal by doping it with nitrogen while growing it by the Czochralski method. It is alleged that oxide precipitate nuclei remaining in wafers sliced from such single crystal after the epitaxial step form oxide precipitates from the early stages of the device step and thus effectively serve as sinks for gettering, so that the effects of IG can be expected from the early stages of the device step.
Later investigations, however, have revealed that high concentration nitrogen doping, for example doping with nitrogen at a concentration not lower than 1xc3x971014 atoms/cm3 results in the formation of oxide precipitate nuclei, which are thermally stable and will not vanish even upon high temperature heat treatment in the epitaxial step, in the wafer inside and near the wafer surface as well and, therefore, in the epitaxial step, stacking faults, dislocations and the like occur in the epitaxial layer with the thermally stably oxide precipitate nuclei formed near the wafer surface serving as origins, readily inducing epitaxial defects. The epitaxial defects cause an increase in the leakage current and degradation in the gate oxide integrity, among others.
On the other hand, when nitrogen doping is effected at a low concentration, for example at 5xc3x971013 nitrogen atoms/cm3 or below, the problem of epitaxial defects development is not encountered but the effect of nitrogen addition to increase the thermal stability of oxide precipitate nuclei is insufficient, so that even those oxide precipitate nuclei occurring inside the wafer disappear upon high temperature heat treatment in the epitaxial step, hence oxide precipitates are scarcely formed upon later high temperature heat treatment in the device step; the IG effect is thus low.
Accordingly, it is an object of the present invention in a first aspect thereof to provide a method of manufacturing epitaxial wafers by which the thermal stability of oxide precipitates is retained even after the epitaxial step without requiring any additional heat treatment step following silicon single crystal pulling up and thus epitaxial wafers capable of exhibiting high IG capability are provided.
Another object of the present invention, in a second aspect thereof, is to provide a method of manufacturing epitaxial wafers which solves the problem mentioned above concerning epitaxial defects resulting from doping with nitrogen and which provides epitaxial wafers allowing formation of a sufficient amount of oxide precipitates upon high temperature heat treatment in the device step while preventing the generation of epitaxial layer defects even when they are manufactured from a silicon single crystal grown with nitrogen doping.
The present invention, which has been completed from such viewpoints, is directed to a method of manufacturing epitaxial wafers in accordance with the first aspect thereof, a method of manufacturing epitaxial wafers in accordance with the second aspect thereof, and a method of producing silicon single crystals controlled with respect to the thermal history in the step of pulling up, as described below.
1. Method of Manufacturing Epitaxial Wafers in Accordance with the First Aspect of the Present Invention (Without Nitrogen Doping)
To investigate the influences of the rate of cooling in the step of silicon single crystal pulling up in the Czochralski method, namely the thermal history of the crystal on the thermal stability of oxide precipitate nuclei formed therein, the present inventors made experiments using silicon single crystals having a diameter of 4xe2x80x3 and changing the rate of pulling up at an intermediate stage of the process.
The concrete method of experimentation was as follows. A cylindrical portion is formed to a length of 500 mm at a pulling rate of 1.0 mm/min and at the time of arrival at a length of 500 mm, the rate of pulling up is changed to 0.5 mm/min, 1.6 mm/min or 2.0 mm/min and the crystal is grown to a length of 550 mm. Thereafter, the rate of pulling up is again returned to 1.0 mm/min and the crystal is grown to 850 mm and then the pulling up is finished by tailing.
After the reduction in pulling rate, the thus-grown single crystal is cooled slowly from the temperature at the start of rate reduction to the lower temperature side by a temperature range of about 100xc2x0 C. and, when the pulling rate is increased, it is rapidly cooled from the temperature at the start of rate increase to the lower temperature side by a temperature range of about 100xc2x0 C. From that portion of each single crystal which has been cooled within the temperature range of 1400-600xc2x0 C., samples are sliced and subjected to treatment at 1100xc2x0 C.xc3x9716 hours (high temperature heat treatment). The thermal stability of oxide precipitates formed by pulling up by the Czochralski method is evaluated by counting the defects induced by the heat treatment.
FIG. 1 is a graphic representation of the relationship between the etch pit density after heat treatment in the experiments involving changes in rate of pulling up in the intermediate stage of the Czochralski method and the temperature at the start point of each changed rate. In FIG. 1, A crystal (slow cooling) corresponds to the case where the pulling rate was changed from 1.0 mm/min to 0.5 mm/min, B crystal (rapid cooling) to the case where it was changed from 1.0 mm/min to 1.6 mm/min, and C crystal (rapid cooling) to the case where it was changed from 1.0 mm/min to 2.0 mm/min.
From the results shown in FIG. 1, it is evident that rapid cooling in the temperature range of 1200xc2x0 C. to 1050xc2x0 C. results in marked increases in the etch pit density after heat treatment, namely in the density of oxide precipitates. Comparison between B crystal and C crystal reveals an increased defect density in C crystal cooled more rapidly. It is also seen that slow cooling in the temperature range of 1000xc2x0 C. to 700xc2x0 C. also results in stable increases in the oxide precipitate density.
The present invention, which has been completed based on the findings obtained in the above-mentioned pulling rate changing experiments by the Czochralski method, is directed to a method of manufacturing epitaxial wafers from a silicon single crystal pulled up by the Czochralski method while controlling the thermal history thereof. More particularly, the present invention includes (1) the first and (2) the second method of producing silicon single crystals as mentioned below and to the method of manufacturing epitaxial wafers which uses silicon wafers sliced from such single crystals, as mentioned below.
(1) The First Single Crystal and the Manufacture of Epitaxial Wafers Using the Same
The first single crystal is produced by selecting a cooling rate of not less than 7.3xc2x0 C./min within the temperature range of 1200-1050xc2x0 C. in crystal pulling up in the Czochralski method and the method of manufacturing epitaxial wafers according to the present invention is characterized in that an epitaxial layer is grown on the surface of silicon wafers sliced from that single crystal.
The reason why the temperature range of 1200-1050xc2x0 C. is selected for such cooling in pulling up the first single crystal is that, as is evident from the above-mentioned results shown in FIG. 1, rapid cooling in this temperature range can result in an increased etch pit density after heat treatment and thus in an increased density of oxide precipitates. As a result, an excellent IG effect can be produced.
Further, rapid cooling, namely a cooling rate of not less than 7.3xc2x0 C./min, is required since this is the cooling rate giving B crystal as evidenced by the pulling rate changing test mentioned above. It has been established that such cooling rate can produce sufficient cooling effects. Furthermore, it is desirable that rapid cooling be carried out at a cooling rate of not less than 8.5xc2x0 C./min, which gives C crystal.
By rapidly cooling at a rate of not less than 7.3xc2x0 C./min within the temperature range of 1200-1050xc2x0 C. during the first single crystal pulling up, vacancies introduced at the solid-liquid interface can be prevented from aggregation or cohesion and thus the density of residual vacancies can be maintained at a high level. As a result, the free energy for the formation of oxide precipitate nuclei decreases and the growth of oxide precipitate nuclei starts within a higher temperature range than in the prior art. Therefore, the thermal stability of oxide precipitate nuclei themselves increases and, thus, a sufficient number of oxide precipitates can be formed even in the heat treatment in the device process following epitaxial growth.
Therefore, according to the method of manufacturing epitaxial wafers which uses the first single crystal, the effects of IG can fully be produced from early stages of the device step by causing an epitaxial layer on the surface of silicon wafers sliced from the silicon single crystal produced by controlling the thermal history thereof, as mentioned above, without any additional heat treatment step prior to the epitaxial step.
(2) The Second Single Crystal and the Method of Manufacturing Epitaxial Wafers Using the Same
The second single crystal is a silicon single crystal produced by the Czochralski method by selecting a cooling rate of not less than 7.3xc2x0 C./min in the temperature range of 1200-1050xc2x0 C. in the step of pulling up and a cooling rate of not more than 3.5xc2x0 C./min in the temperature range of 1000-700xc2x0 C. The method of manufacturing epitaxial wafers according to the present invention is characterized in that an epitaxial layer is grown on the surface of silicon wafers sliced from that single crystal.
The rapid cooling in the temperature range of 1200-1050xc2x0 C. in the first stage of the cooling step to be controlled in pulling up the second single crystal produces the same effects as the cooling of the first single crystal. Further, the subsequent slow cooling in the temperature range of 1000-700xc2x0 C. in the next stage is based on the finding from the results shown in FIG. 1 that the oxide precipitate nuclei formed can be grown and rendered more thermally stable by such slow cooling.
Within the temperature range of 1000-700xc2x0 C., a slow rate of cooling of not more than 3.5xc2x0 C./min is employed, as mentioned above, since as indicated by the results of the pulling rate changing test mentioned above, such cooling rate produces sufficient effects of slow cooling to stably increase the density of oxide precipitates.
Even in the method of manufacturing epitaxial wafers which uses the second single crystal, when an epitaxial layer is grown on the surface of silicon wafers sliced from the silicon single crystal produced by controlling the thermal history in the manner mentioned above, the oxide precipitate nuclei will not shrink or vanish upon epitaxial treatment without needing any additional heat treatment step, like in the production method which uses the first single crystal.
It is desirable that the oxygen content in the silicon wafers sliced from the first or second single crystal be not less than 12xc3x971017 atoms/cm3 (ASTM ""79). Silicon single crystals produced by the Czochralski method generally contain about 1018 atoms/cm3 of supersaturation oxygen but, when the oxygen content is insufficient, a marked decrease in wafer strength may result and/or sufficient IG effects will not be produced in certain instances. Therefore, it is desirable that the oxygen concentration be not less than 12xc3x971017 atoms/cm3 (ASTM ""79) so that the stability of oxide precipitate nuclei can be effectively secured when the first or second single crystal is used.
2. The Second Method of Manufacturing Epitaxial Wafers (with Nitrogen Doping)
The present inventors then carried out experiments in which the pulling rate was changed in the intermediate stage in the Czochralski method using silicon single crystals having a diameter of 6xe2x80x3 to investigate the influences of the thermal history of the crystal on the thermal stability of oxide precipitate nuclei formed in the single crystal doped with nitrogen.
To be concrete, the experiments were carried out as follows. A cylindrical portion is grown to a length of 500 mm at a pulling rate of 0.7 mm/min and, after attaining the length of 500 mm, the pulling rate is slowed down to 0.2 mm/min or increased to 1.2 mm/min, followed by growing to a length of 550 mm. Thereafter the pulling rate is returned to 0.7 mm/min and, after growing to 850 mm, the pulling up procedure is finished by tailing. The dopant nitrogen concentration in the single crystal is adjusted to 1xc3x971013 atoms/cm3.
The single crystal thus grown is cooled slowly or rapidly according to the change in pulling rate, as mentioned above. After pulling up, samples are sliced from that portion cooled within the temperature range of 1400-800xc2x0 C., subjected to high temperature heat treatment, namely treatment at 1100xc2x0 C. for 16 hours and examined for the number of heat treatment-induced defects, and the thermal stability of oxide precipitates formed during pulling up by the Czochralski method is evaluated in terms of the oxide precipitate density.
FIG. 2 is a graphic representation of the relationship between the etch pit density after heat treatment and the temperature at the start of pulling rate changing as revealed by the experiments involving pulling rate changing in the intermediate stage of the Czochralski method. From the results shown in FIG. 2, it is evident that rapid cooling in the temperature range of 1150-1020xc2x0 C. results in an increased etch pit density after heat treatment and in an increased density of oxide precipitates. It is also evident that slow cooling in the temperature range of 1000-850xc2x0 C. also results in a stable increase in oxide precipitate density.
The inventors further carried out experiments involving detaching and rapid cooling in the Czochralski method to find out the temperature range in which vacancies, which are void-type defects, and oxide precipitate nuclei are formed during the growth of nitrogen-doped single crystals. Concretely, after formation of a predetermined length of a cylindrical portion, the pulling up is discontinued for a predetermined period and, thereafter, the single crystal is detached from the melt and examined for the density of defects (cf. Japanese Patent Application Laid-Open (Kokai) No. H10-236897, if necessary).
In the above experiments, silicon single crystals having a diameter of 6xe2x80x3 are grown with doping with nitrogen in a concentration of 1xc3x971014 atoms/cm3 to thereby render the defect forming temperature for crystal growth conspicuous. The experimental conditions were as follows. In the step of cylindrical portion growth at a pulling rate of 1.0 mm/min, the crystal growth is discontinued for 5 hours and then the crystal is detached from the melt surface and processed for lengthwise division in the direction of single crystal puling up, and the density of void defects generated is determined. Void defects are detected using a Bio-Rad defect detector OPP (optical precipitate profiler) and the density thereof is evaluated.
FIG. 3 is a graphic representation of the relationship between the OPP defect density and the crystal detachment temperature as revealed in the detaching and rapid cooling experiments by the Czochralski method. The results shown in FIG. 3 indicate that the temperature range in which void defects are readily formed is 1100-1020xc2x0 C. For the case of no nitrogen addition, it is reported in the art that the void defect forming temperature range is 1100-1070xc2x0 C. (H. Nishikawa, T. Tanaka, Y. Yanase, M. Hourai, M. Sanno and H. Tsuya, Jpn. J. Appl. Phys. Vol. 36 (1997), p. 6595). The above results, however, show that when doping is carried out with nitrogen, the defect formation temperature range widens to the lower temperature side by about 50xc2x0 C. Thus, it is indicated that the diffusion of vacancies is inhibited by the addition of nitrogen and the degree of supersaturation of vacancies is still high even at below 1070xc2x0 C. and, as a result, void defects are formed.
Then, to reveal the oxide precipitate nuclei formation temperature, samples obtained from the detaching rapid cooling experiments were subjected to constant temperature heat treatment at 1050xc2x0 C. for 4 hours and then etched, and the density of such nuclei was determined.
FIG. 4 is a graphic representation of the relationship between the etch pit density and the temperature at detachment as revealed in the detaching rapid cooling experiments by the Czochralski method. From the results shown in FIG. 4, it is evident that etch pits are observable at detachment temperatures not higher than 1050xc2x0 C. They are particularly outstanding when the detachment temperature is within the range of 1050-800xc2x0 C. This fact indicates that stable oxide precipitate nuclei are formed in the temperature range of not higher than 1050xc2x0 C. during the process of cooling for crystal growth.
While, in the art, it has so far been considered that, for nitrogen-free crystals, the formation of oxide precipitate nuclei is observed in the detachment temperature range of 900xc2x0 C. and below, the above results indicate that the addition of nitrogen raises the temperature range in which oxide precipitate nuclei are formed. This is due to the fact that the number of residual vacancies increases under the influence of nitrogen addition and the free energy for the formation of oxide precipitate nuclei lowers and, as a result, the formation of oxide precipitate nuclei in the higher temperature range becomes possible.
The second epitaxial wafer manufacturing method of the present invention has been completed based on the above findings obtained in the pulling rate changing experiments and detaching and rapid cooling experiments by the Czochralski method. In particular, the present invention has been completed based on the finding that there is an appropriate rate of cooling according to the dopant nitrogen concentration at which rate single crystals high in oxide precipitate density can be produced. More specifically, the present invention is directed to methods of producing (1) the third to (4) sixth single crystals mentioned below and to methods of manufacturing epitaxial wafers using silicon wafers sliced from the respective single crystals.
(1) The Third Single Crystal and the Production of Epitaxial Wafers Using the Same
The method of producing the third single crystal is characterized in that, in pulling up a silicon single crystal doped with 1xc3x971012 atoms/cm3 to 1xc3x971014 atoms/cm3 of nitrogen in the Czochralski method, a cooling rate of not less than 2.7xc2x0 C./min is employed in the single crystal temperature range of 1150-1020xc2x0 C.
In the third single crystal, the dopant nitrogen concentration is restricted to 1xc3x971012 atoms/cm3 to 1xc3x971014 atoms/cm3 because the effect of thermally stabilizing oxide precipitate nuclei is low, hence those oxide precipitate nuclei which occur within the wafer inside disappear upon high temperature heat treatment in the epitaxial step, when the nitrogen concentration is lower than 1xc3x971012 atoms/cm3, while when the concentration is higher than 1xc3x971014 atoms/cm3, the effect of thermally stabilizing oxide precipitate nuclei is conversely excessive, hence those oxide precipitate nuclei having high thermal stability and occurring in the vicinity of the wafer surface will not vanish even when subjected to high temperature heat treatment in the epitaxial step but rather cause generation of epitaxial layer defects.
The temperature range for the above cooling is restricted to 1150-1020xc2x0 C. since, as is evident from the results shown in FIG. 2, the etch pit density after heat treatment can increase and the oxide precipitate density can increase accordingly when rapid cooling is carried out in that temperature range. Further, the rate of cooling is restricted to 2.7xc2x0 C./min or more since this is a cooling rate found securable and appropriate in the above pulling rate changing test mentioned above; it has been confirmed that sufficient cooling effects can be produced at such a rate.
Thus, in the method of producing the third single crystal, the nitrogen concentration is restricted to a relatively low level within the range of 1xc3x971012 atoms/cm3 to 1xc3x971014 atoms/cm3 to thereby suppress the formation of thermally stable oxide precipitate nuclei and, on the other hand, the single crystal is rapidly cooled in the temperature range of 1150-1020xc2x0 C. to thereby prevent vacancies incorporated at the solid-liquid interface from cohesion and cause the formation of oxide precipitate nuclei on the higher temperature side and thus allow oxide precipitate nuclei to grow. By this, it is possible to prevent epitaxial layer defects from being generated and attain sufficient oxide precipitate formation even in the heat treatment after epitaxial growth.
In accordance with the second epitaxial wafer manufacturing method, an epitaxial layer is grown on the surface of silicon wafers sliced from the third silicon single crystal. By this, it is possible to manufacture epitaxial wafers high in IG capability with scarce occurrence of epitaxial layer defects. In other words, it is possible to manufacture high-quality epitaxial wafers free of defects in the device active region.
(2) The Fourth Single Crystal and the Method of Manufacturing Epitaxial Wafers Using the Same
The method of producing the fourth single crystal is characterized in that, in pulling up the silicon single crystal doped with 1xc3x971012 atoms/cm3 to 1xc3x971014 atoms/cm3 in the Czochralski method, a cooling rate of not more than 1.2xc2x0 C./min is employed in the single crystal temperature range of 1000-850xc2x0 C.
Here, slow cooling is employed in the temperature range of 1000-850xc2x0 C. because, as is evident from the results shown in FIG. 2, the growth of oxide precipitate nuclei can be promoted even by slow cooling in that temperature range and thereby oxide precipitate nuclei relatively large in size can be formed. Based on the results of the above-mentioned pulling rate changing test, a slow cooling rate of not more than 1.2xc2x0 C./min is employed as the cooling rate in the above restricted temperature range; sufficient oxide precipitate nuclei growth can be attained at such a cooling rate.
In accordance with the second epitaxial wafer manufacturing method, an epitaxial layer is grown on the surface of silicon wafers sliced from the fourth silicon single crystal, whereby epitaxial wafers exhibiting high IG capability can be manufactured with scarce occurrence of epitaxial layer defects.
(3) The Fifth Single Crystal and the Method of Manufacturing Epitaxial Wafers Using the Same
The method of producing the fifth single crystal is characterized in that, in pulling up the silicon single crystal doped with 1xc3x971012 atoms/cm3 to 1xc3x971014 atoms/cm3 of nitrogen in the Czochralski method, a cooling rate of not less than 2.7xc2x0 C./min is employed in the single crystal temperature range of 1150-1020xc2x0 C. and, thereafter, a cooling rate of not more than 1.2xc2x0 C./min is employed in the temperature range of 1000-850xc2x0 C.
The fifth single crystal can be produced by combining the method of producing the third single crystal with the method of producing the fourth single crystal.
In accordance with the second epitaxial wafer manufacturing method, an epitaxial layer is grown on the surface of silicon wafers sliced from the fifth silicon single crystal, whereby epitaxial wafers exhibiting high IG effects can be produced with scarce occurrence of epitaxial defects, as in the case where the third or fourth single crystal is used.
(4) The Sixth Single Crystal and the Method of Manufacturing Epitaxial Wafers Using the Same
The method of producing the sixth single crystal is characterized in that, in pulling up the silicon single crystal doped with 5xc3x971013 atoms/cm3 to 1xc3x971016 atoms/cm3 of nitrogen in the Czochralski method, a cooling rate of not less than 6.5xc2x0 C./min is employed in the single crystal temperature range of 1150-800xc2x0 C.
From the viewpoint of preventing epitaxial layer defects from occurring, it is effective to select a low nitrogen concentration, as mentioned hereinabove. However, an increased nitrogen concentration thermally stabilizes those oxide precipitate nuclei which occur within the wafer and prevents them from vanishing upon high temperature heat treatment to thereby contribute toward the increase in oxide precipitate density, hence it is effective to utilize such increased nitrogen concentration.
The present inventors conjectured that even thermally stable oxide precipitate nuclei occurring near the wafer surface, if not large in size, would vanish upon high temperature heat treatment for epitaxial growth and have now arrived at the method of producing the sixth single crystal.
For the sixth single crystal, a relatively high nitrogen concentration is selected within the dopant nitrogen concentration range of 5xc3x971013 atoms/cm3 to 1xc3x971016 atoms/cm3 to cause the formation of thermally stable oxide precipitate nuclei. And, by rapidly cooling at a rate of not less than 6.5xc2x0 C./min in the temperature range of 1150-800xc2x0 C., namely by cooling as rapidly as possible in the temperature range of 1150-800xc2x0 C., which corresponds to the temperature range in which void defects are formed (cf. FIG. 3) and the temperature range in which oxide precipitate nuclei are grown (cf. FIG. 4), it is intended that the formation of void defects be supposedly causative of epitaxial defects and the growth of thermally stable oxide precipitate nuclei be both prevented.
The upper limit to the nitrogen concentration for doping the sixth single crystal is set at 1xc3x971016 atoms/cm3 because, at a level exceeding this limit, single crystals have dislocations in single crystal growing and the product yield thus markedly decreases. The lower limit to the cooling rate is 6.5xc2x0 C./min (rapid cooling) since, at a lower rate, the effect of preventing voids from cohesion is slight and the effect of suppressing the growth of oxide precipitate nuclei is also slight due to the high dopant nitrogen concentration, with the result that thermally stable oxide precipitate nuclei large in size are formed.
In accordance with the second epitaxial wafer manufacturing method, an epitaxial layer is grown on the surface of silicon wafers sliced from the sixth silicon single crystal, whereby epitaxial wafers exhibiting high IG capability can be manufactured with scarce occurrence of epitaxial layer defects, as in the case where one of the third to fifth single crystals is used. In other words, it is possible to manufacture high-quality epitaxial wafers free of defects in the device active region.
In the second epitaxial wafer manufacturing method, it is desirable that the oxygen content in the above-mentioned third to sixth single crystals be not less than 4xc3x971017 atoms/cm3 (ASTM ""79) so that a sufficient amount of oxide precipitates to produce sufficient IG effects can be secured. The reason is that while the silicon single crystals produced by the Czochralski method generally contain about 1018 atoms/cm3 of supersaturation oxygen, as mentioned above, an insufficient oxygen concentration results in decreases in wafer strength and/or failure to produce satisfactory IG effects.
The method of doping to be employed in the second epitaxial wafer manufacturing method may be any one provided that doping with a predetermined concentration of nitrogen can be realized. Thus, for example, there may be mentioned the method which comprises incorporating a nitride into the raw material or melt, incorporating a nitrogen-added silicon crystal by the floating zone (FZ) technique or a wafer having a silicon nitride film on the surface into the wafer material, growing single crystals while passing nitrogen gas or a nitrogen compound gas through the furnace, blowing nitrogen or a nitrogen compound gas against polycrystalline silicon at high temperatures prior to melting, or using a nitride-made crucible.